An ideal circularly symmetric "single mode" optical fiber can support two independent, degenerate modes of orthogonal polarization. Either one of these constitutes the fundamental HE.sub.11 mode. In general, the electric field of light propagating along the fiber is a linear superposition of these two polarization cigenmodes.
In practical single mode fiber, various imperfections such as asymmetrical lateral stress and a non-circular core typically break the circular symmetry of the ideal fiber and lift the degeneracy of these two polarization modes. The two modes then propagate with different phase velocities, and this difference between their effective refractive indices is called birefringence.
Fiber birefringence can result from either a geometrical deformation or from various elasto-optic, magneto-optic or electro-optic index changes. In so-called polarization-preserving fibers asymmetry is deliberately introduced into the fiber. However, in ordinary (non-polarization-preserving) fibers the birefringence mechanisms act on the fiber in substantially unpredictable manner. Thus, the polarization state of the guided light will typically evolve through a pseudorandom sequence of states along the fiber, with the polarization state at the fiber output typically being both unpredictable and unstable. On average, a given polarization state in a given fiber is reproduced after a certain length L.sub.p, the polarization "beat" length associated with the given fiber.
The presence of birefringence in conventional single mode fiber results in signal dispersion (so-called polarization mode dispersion or PMD) and thus typically is undesirable, especially for applications that involve high bit rates or analog transmission (e.g., for optical fiber analog CATV systems).
It is known that fiber having low PMD can be produced by rapidly spinning the preform while pulling the fiber from the preform. The prior art teaches that this results in periodically interchanged fast and slow birefringence axes along the fiber, which can lead to very low net birefringence due to piecemeal compensation of the relative phase delay between the polarization eigenmodes, provided the spin pitch is much less than the "un-spun" fiber beat length. See, for instance, A. Ashkin et al., Applied Optics, Vol. 20(13), p. 2299; A. J. Barlow et al., Applied Optics, Vol. 20(17), p. 2962; and S. C. Rashleigh, Laser Focus, May 1983.
It is primarily the prior art requirement that the spin pitch be much less than the "unspun" L.sub.p which makes the prior art technique substantially unsuitable for current commercial fiber production. For instance, assuming that the unspun L.sub.p is about 1 m and the draw speed is 10 m/seconds, then the preform has to be spun at 6000 rpm in order to yield a spin pitch that is 1/10th of the unspun L.sub.p. This is typically not practical in commercial fiber production.
In view of the commercial significance of low birefringence optical fiber, it would be highly desirable to have available a technique for producing such fiber that is compatible with current commercial practice, e.g., that is usable even at the high draw speeds that are typically used now. This application discloses such a technique.